The immune system acquired the mechanism that can respond to various foreign antigens. The mechanism brings about the diversity of an antigen receptor by recombination of V, (D) and J fragment in T cells and B cells. Although this mechanism brought a result that produces self-reactive lymphocytes simultaneously, these self-reactive lymphocytes are removed by the negative selection in the thymus or bone marrow, and are further controlled by the self-tolerance mechanism of clone removal or anergy in the periphery.
Although it is thought that an autoimmune disease is developed by the breakdown of self-tolerance, the researches using various disease model mice have been made towards the elucidation of the mechanism of pathogenesis of the disease. However, there are many still unknown matters about the etiology of an autoimmune disease and the molecular mechanism of self-tolerance. In such a situation, existence of the mouse which shows the symptoms of an autoimmune disease with a single gene deficit is very important, when trying to study the etiology of an autoimmune disease from a molecular biological viewpoint. CTLA4−/−mouse which causes lethal systemic lymphocytes infiltration (Waterhouse P. et. al., Science 270:985˜988, 1995, Tivol E. A. et. al., Immunity 3:541˜547, 1995), SHP-1 deficit mothaten mice (Shulltz L. D. et. al., Cell 73: 1445˜1454, 1993), TGF-beta-1 knockout mouse (Shull M. M. et. al., Nature 359:693-699, 1992), lyn−/−mouse which shows the symptoms of glomerular nephritis (Hibbs M. L. et. al., Cell 83:301-311, 1995), and FCRIIB−/−mouse (Bolland S. & Ravetch J. V., Immunity 13:277-285, 2000) are the representation, and the relation of these molecules and self-tolerance is studied.
PD-1 is type I membrane protein of 55 kD belonging to an immunoglobulin family. Bothmouse PD-1 and human PD-1 consist of 288 amino acids, and have signal peptide at N terminal (20 amino acid) and hydrophobic region in the middle part, which is a transmembrane region (The EMBO J. 11 (11):3887-3895, 1992); Japanese patent Publication No. 5-336973; EMBL/GenBank/DDJB Acc. No. X67914, Genomics 23:704, 1994; Japanese patent Publication No. 7-291996 (U.S. Pat. No. 5,629,204).
In a thymocyte, PD-1 is expressed during a CD4−CD8− cell differentiation to a CD4+ CD8+ cell (Nishimura H. et. al., Int. Immunol. 8:773-780, 1996, Nishimura H. et. al., J. Exp. Med. 191:891-898, 2000). Moreover, in the periphery, PD-1 is expressed in T cells and B cells which were activated by the stimulus from an antigen receptor (Agata Y. et. al., Int. Immunol. 8:765-772, 1996), and in bone marrow cells including activated macrophage.
PD-1 has ITIM (Immunoreceptor tyrosine-based inhibitory motif) in its intracellular region, therefore it is considered to be a negative regulator in immune reaction. PD-1 deficit mice develop a lupus-like autoimmune disease such as glomerular nephritis and arthritis (C57BL/6 genetic background) (Nishimura H. et. al., Int. Imuunol. 10:1563-1572, 1998, Nishimura H. et. al., Immunity 11:141-151, 1999) and dilated cardiomyopathy-like disease (BALB/c genetic background) (Nishimura H. et. al., Science 291:319-322, 2001), it became clear that PD-1 is a regulator of the development of autoimmune disease, especially one of self-tolerance in the periphery.